Introduction: Magnetic resonance imaging of the breast has been heralded as a significant advance in the diagnosis of breast disease, with significantly higher sensitivity for the detection of breast cancer than either physical exam or mammography. As a result, many breast lesions are now being discovered which can only be visualized with MRI. The goal of this project is to develop imaging guidance and biopsy methods which can be used to provide pathologic diagnoses for lesions discovered on breast MRI. Three objectives have been pursued: 1. Image guidance techniques. 2. MR-guided needle localization and preoperative hookwire placement. 3. MR-guided core needle biopsy Methods: Imaging methods for guidance were developed for both closed bore 1.5T conventional magnets and the open 0.5T Signa-SP system at Stanford. Requirements for imaging include rapid scan acquisition, T1-weighting, optional fat-suppression, and minimal needle size artifacts. Short echo-train Fast Spin Echo protocols were implemented for both systems for non-fat-suppressed images At 1.5T, spectral-spatial Fast Spin Echo sequences were written and modified to include magnetization transfer. At 0.5T, 3-point Dixon gradient echo sequences were implemented, and an automated Dixon reconstruction program was developed for rapid image processing and display. MR-guided needle placement was developed using a commercially available open platform coil. Interactive techniques, similar to freehand ultrasound guided breast needle localization procedures were implemented. Procedures were tested on 20 lesions which were visible by other imaging methods, in patients going for excisional biopsy. Results: Successful imaging methods were developed for both 1.5T and 0.5T systems. To date, over 20 needle localizations have been performed with average accuracy of less than 9 mm in all cases. Core biopsy procedures require further refinement to reduce the artifacts cast by biopsy needles on Dixon images, so that they do not obscure target lesions. Conclusions: A rapid, easy, accurate and successful technique for biopsy of lesions detected on Breast MR images has been developed. Future work is in progress to develop core biopsy techniques which will supplant some of the surgical biopsies which are currently required for lesions detected by MR.